CONDUCTORS, INSULATORS AND SEMICONDUCTORS

Electrical conduction is the movement of electric charge through a transmission medium. The movement can form an electric current in response to an electric field. This charge movement depends on the material.

Conduction, under normal conditions, depends on the availability of free and mobile electrons. This explains why metals which have free electrons are very good conductors while such materials as plastic with no free electrons are poor conductors.

For conduction to be achieved, a material should have these free electrons, and if not, they must be made available through increasing temperature or even introducing them through such processes as doping.

Conduction in metals and resistors is well described by Ohm's Law, which states that the current is proportional to the applied electric field.

Conductors, Insulators and Semi-conductors

Materials can be classified as conductors, insulators or semi-conductors depending on their electrical conductivity.

Conductors

Conductors are materials that allow electric current of flow through them. They obey Ohm's law and have very low resistance. They therefore carry electric currents from place to place without dissipating a lot of power. As a result, metals are useful as connecting wires to carry electrical signals from place to place.

One unique property of conductors is that their conductivity decreases with increase in temperature. When conductors are heated, the present free electrons gain more kinetic energy increasing their collision with each other. These collisions instead increases the materials resistivity to electric current flow which requires that the available free electron flow in one specific direction.

Insulators

Insulators are materials that do not allow electric current to flow through them. They do not have free and mobile electrons essential for charge flow, and are therefore highly resistive to current flow. Glass, most polymers (plastics), rubber and wood are all examples of insulators. These are materials which will refuse to carry an electric current.

 They are useful for jobs like coating (‘insulating’) electric wires to prevent them from 'shorting together' or giving you a shock.

Semiconductors

Semiconductors are materials whose conductivity properties lie between those of conductors and insulators. Semiconductors are generally insulators at low temperatures and conductors as temperatures increases.

We can also look at a semiconductor as is given away by its name - it 'conducts a little bit'. A semiconductor will carry electric current, but not as easily as a normal conductor.

Semiconductors do not have   free electrons naturally and we therefore do not expect them to conduct current. Therefore, they are made to conduct by enhancing the availability of free electrons or any other particles and conditions necessary for conduction. Heating a semiconductor for example, breaks the internal bonds hence freeing some electrons which therefore move freely to conduct current. Other than heating, some impurities can be added to a semiconductor with an aim of introducing free electrons, or holes in some cases, to enable them conduct electric current.

These two methods of making a semiconductor conduct current create two types of semiconductors, the intrinsic semiconductors and the extrinsic semiconductors.

Intrinsic Semiconductors: These are pure (group four elements) semiconductors whose semiconducting properties occur naturally. They can be made to conduct by increasing their temperature. (Read more on Band theory of conduction inn solids)

Extrinsic Semiconductors: These materials are turned into semiconductors by doping them with small amounts of foreign atoms. The number of doping atoms you need to add is very small. If you left all the doping atoms inside the package of a transistor and removed the bulk of the material you'd be left with a vacuum better than exists between the planets of the solar system. (Read more on Doping)